In recent years, a focused-ion-beam (:FIB) device has come into a wide use for the formation of a thin-film sample dedicated for a (scanning) transmission electron microscope (:(S) TEM). In particular, in the formation of a thin-film sample dedicated for semiconductor-device failure analysis, it is wished in almost all cases to stop the FIB fabrication at the location of a specific cross-section of the semiconductor device. Accordingly, there has occurred a prevailing use of a FIB-SEM dual-beam device which allows the FIB-fabricated cross-section to be observed by the scanning electron microscope (:SEM). At this time, it is common to perform the image observation in such a manner that secondary electrons (:SEs), which reflect surface information and topographic information about the sample, are employed as the signal for the observation.
Also, in the SEM, in some cases, the image observation is performed in such a manner that back-scattered electrons (:BSEs), which reflect material information and composition information about the sample, are employed as the signal for the observation. The BSEs are the generic designation for electrons which have emitted from the sample in accompaniment with a wide range of energy distribution from 50 eV to incident energy of the electron beam used in the SEM. Of the BSEs, there are two types of reflected electrons, i.e., reflected electrons which have undergone completely elastic scatterings, and thus exhibit no energy loss, and reflected electrons which have been emitted after undergoing only a few times of mutual interactions with the electron beam inside the sample, and thus exhibit less energy loss. In the present specification, these two types of reflected electrons will altogether be referred to as “low-loss electrons (:LLEs)”. Because of no or less mutual interaction between the electron beam and the sample, the LLEs are regarded as being electrons which have not intruded into the inside of the sample, i.e., electrons which have the surface information about the sample.
Incidentally, the LLE detection in the SEM had been publicized in 1970 by a group whose leader is Oliver C. Wells. In theses and patent specifications presented by Wells et al., the description has been given concerning the acquisition of contrasts including the surface information, material information, and composition information about the sample at a high acceleration voltage in the single-unit SEM.